Sub-surface well pumps of the progressive cavity type are operated to pump well fluids to the surface through a production tubing by a sucker rod string which is rotated at the well head by a suitable drive means. Ths sucker rod string, whose bottom end is connected to the rotor of the pump, comprises a plurality of serially connected sucker rods of relatively great lengths, twenty-five to thirty feet long. Since the well bore and the tubing to whose bottom end the stator of the pump is connected, are not perfectly perpendicular, it is necessary to mount rod guides or centralizers on the shanks of some, if not all, of the rods of the string to prevent or minimize damage or wear to the rods and to the tubing at locations where the rods would otherwise engage and rub against the tubing.
To minimize wear and damage to such rod guides or centralizers and to the tubing, it is desirable that the sucker rods on which the centralizers are mounted be relatively free to rotate relative to the centralizers if and when the centralizers are forced into engagement with the internal surfaces of the tubing so that the centralizers not rotate relative to the tubing and rub there against.
It is also desirable that the centralizers not rub against the metal of the sucker rods themselves since such contact would remove the corrosion preventing or inhibiting coating with which the rods are protected from the corroding effects of the well fluids being pumped and thus expose the bare metal of the rods to the well fluids.
The need for such rod guides or centralizers is well known and various devices, as disclosed in the U.S. Pat. Nos. 31,016 to Clarence Oster and 1,605,316 to R. A. Wilson, have been designed but such prior art devices are not mountable at one or more selected locations on the shank of a sucker rod and moreover expose metal surfaces of the string to the well fluids.